The present invention relates to a photo mask used in fabrication of a semiconductor device, and more particularly to, a method of correcting an error of a phase difference in a phase shift mask.
In general, a semiconductor device includes various patterns and a photolithography process is used to form these patterns. In accordance with this photolithography process, a photoresist layer of which solubility is changed by irradiation of light is formed on a layer to be patterned on a wafer, a predetermined portion of the photoresist layer is exposed by a photo mask and then the portion of which solubility to a developing solution has been changed or not changed is removed to form a photoresist layer pattern. After that, the layer to be patterned by the photoresist layer pattern is removed through an etch to form a pattern.
However, a pattern sizes have recently been reduced as an integration degree of semiconductor devices has increased, and accordingly, a pattern defect due to a lowering in resolution and so on becomes a serious problem in the formation of the pattern using photolithography.
Accordingly, various resolution enhancement techniques capable of enhancing the resolution have been continuously developed, and among these techniques, there is a method of minimizing the pattern defect by increasing the resolution using a phase shift mask instead of an existing binary mask.
FIG. 1 is a sectional view illustrating an example of a conventional phase shift layer pattern. Referring to FIG. 1, a phase shift layer 100 has a structure in that a phase shift layer 120 is disposed, as a phase shift pattern, on a light transmitting substrate 110 such as quartz. When performing an exposure process using the phase shift mask 100, there is a phase difference of a certain degree, e.g. 180 degrees, between a light 131 transmitted through a non-phase shift region by which the light transmitting substrate 110 is exposed and a light 132 transmitted through a phase shift region on which the phase shift layer 120 is not exposed. This phase difference is generated due to a difference in a refractive index in the light transmitting substrate 110, the phase shift layer 120 and air. This is because a wavelength of the light 132 transmitted through the phase shift region is reduced when the light 132 is transmitted through the phase shift layer 120 having a relatively large refractive index, and as a result, the phase of the light 132 is inverted by 180 degrees. This phase difference causes a destructive interference in a boundary between the phase shift region and the non-phase shift region and a resolution in the pattern formation process is increased by this destructive interference.
Accordingly, to obtain an effect of the resolution increase, it is important that the phase difference of 180 degrees is shown between the light 131 transmitted through the non-phase shift region and the light 132 transmitted through the phase shift region in the process of fabricating a phase shift mask. As described above, the phase difference is dependent on the refractive indexes of the light transmitting substrate 110 and the phase shift layer 120 and is also dependent on thicknesses of the light transmitting substrate 110 and the phase shift layer 120. Accordingly, in a design of the phase shift mask, the thicknesses and refractive indexes of the light transmitting substrate 110 and the phase shift layer 120 are selected to be capable of obtaining a desired phase difference.
However, errors can be generated in the thicknesses of the light transmitting substrate 110 and the phase shift layer 120 in the process of fabricating a phase shift mask. In one example, to fabricate the phase shift mask, an etch process removes some portion of a phase shift material layer after the phase shift material layer for formation of the phase shift layer 120 on the light transmitting substrate 110. However, in this etch process, a portion of the light transmitting substrate 110 can be etched to partially reduce the thickness of the light transmitting substrate 110. Also, in the etch process, an upper face of the phase shift layer 120 can be etched to reduce the thickness of the phase shift layer 120. By influence in this etch process, when the thicknesses of the light transmitting substrate 110 and the phase shift layer 120 are changed without changing their refractive indexes, there is generated a phase difference error in which the phase difference between the light 131 transmitted through the non-phase shift region and the light 132 transmitted through the phase shift region is larger or smaller than 180 degrees, and particularly, the phase difference error over a limit error does not allow an effect of a desired resolution decrease.
Accordingly, in the prior art, when such a phase difference error is generated, a method has been used for correcting the phase difference error by implementing an additional etch process on the light transmitting substrate 110 and/or the phase shift layer 120 and adjusting the total thickness again. However, a critical dimension of the phase shift layer 120 can be damaged by the additional etch process and an overall critical dimension uniformity can also be decreased. Also, a defect due to an etch byproduct can be generated.